It in known to process a set of one or more audio input signals for playback through headphones such that the listener has the impression of listening to sounds from a plurality of virtual speakers located at pre-defined locations in a listening room. Such processing is called spatialization and binauralization herein. The filters that process the audio input signals are called binaural filters herein. If not for such processing, a listener listening through headphones would have the impression that the sound was inside that listener's head. The audio input signals may be a single signal, a pair of signals for stereo reproduction, a plurality of surround sound signals, e.g., four audio input signals for 4.1 surround sound, five audio input signals for 5.1, seven audio input signals for 7.1, and so forth, and further might include individual signals for specific locations, like of a particular source of sound. There is a pair of binaural filters for each audio input signal to be spatialized. For realistic reproduction, the binaural filters take into account the head related transfer functions (HRTFs) from each virtual speaker to each of a left ear and right ear, and further take into account both early echoes and the reverberant response of the listening room being simulated.
Thus it is known to pre-process signals by binaural filters to produce a pair of audio output signals—binauralized signals—for listening through headphones.
It is often the case that one wishes to listen to binauralized signals through a single speaker, that is, monophonically by electronically downmixing the signal for monophonic reproduction. An example is listening through a monophonic loudspeaker in a mobile device. It often also is the case that one wishes to listen to such sounds through a pair of closely spaced loudspeakers. In that latter case, the binauralized output signals are also mixed down, but by audio crosstalk rather than electronically. In both cases, the binauralized then mixed down signal sounds unnatural, in particular sounds reverberant with reduced intelligibility and audio clarity. It is difficult to eliminate this problem without compromising the impression of space and distance in the binauralized audio.